A varistor-polymer composite with nonlinear electrical-thermal switching properties

We studied the dependence of the E-J characteristics on the sintering conditions in varistor fillers, the filler content in the polymer matrix and for different polymers and suggest a possible nonlinear electrical-thermal switching mechanism. The nonlinear coefficient and breakdown field of a sample with 100 vol.% varistor fillers sintered at 950 degrees C for 5 h were 12.24 and 135 V/mm, separately. A decrease in the amount of filler content in polymeric matrix or using a fortified polymer improved the electrical properties by increasing the breakdown field to the 263-964 V/mm range and decreasing the leakage current density in the 1.06 x 10(-5) to 1.13 x 10(-10) A/cm(2) range. This varistor-polymer composite can exhibit two nonlinearities in electrical conductivity. In the conducting state above the varistor filler breakdown, the polymer matrix reduces the conductivity, if a critical temperature is reached. Thereby the over voltage is limited by the fillers (i.e. varistor effect) and the over current is limited by the polymer matrix (i.e. positive temperature coefficient effect). (C) 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved.